Reading in low light can cause eyestrain, it's just that eyestrain isn't as permanent or serious as most people think it is. It'll go away by itself when you give your eyes some rest. It's just uncomfortable.

The reading = myopia thing is also a myth, though failing to exercise your long distance vision enough can weaken it (which has been observed in people who spend a long time aboard submarines, for instance), so as with most things in life, balance is key.

Anyway, I tried reading in slightly darker conditions last night and it was very comfortable at a 7. I'm sure the lowest light settings would be ideal if I was out of the city, so I wouldn't really want to raise the minimum brightness any. The current scale seems to cover most eventualities really well; I had it on maximum in bright sunlight the other day and it was definitely easier to read in such bright conditions than my KK was.